The present invention relates to the purification of salt crystals and, more particularly, the removal of insoluble impurities as well as soluble impurities from salts such as potassium chloride, sodium chloride, sodium sulfate and sodium carbonate.
In practice, potassium chloride, sodium chloride and the like are produced by mining operations which produce a raw salt product containing a variety of impurities thereby requiring further processing before use. As the chemical raw material used in the production of a variety of other chemicals, these salts must meet certain purity specifications depending upon the specific chemical process with which they are to be utilized. Of particular importance in providing a satisfactory salt product to the user are the calcium and magnesium compounds. These soluble impurities appear usually in the form of sulfates and chlorides and are known to have an adverse effect on a host of different industrial processes.
The purification of sodium chloride salt can be accomplished by dissolving the mined salt together with its soluble impurities to form a brine, separating the brine from these insoluble impurities liberated as a result of dissolving the salt, and recovering the salt by crystallization processes. These steps result in an increased purity of the crystallized product commensurate with the purity of the brine, it being understood that continued removal of the desired salt by crystallization increases the concentration of soluble impurities in the brine until some of these become insoluble, and which then become incorporated, encapsulated or occluded within the resultant salt crystals along with portions of the impure brine.
The purification of other salts has been primarily directed to the removal of impurities accomplished by a number of different leaching processes in which the ore is contacted with a leaching solution which becomes enriched with the product being recovered and then subjected to further processing such as crystallization. Other attempts to purify salt have considered selective precipitation wherein a chemical is added to a salt solution or brine to precipitate only the particular material of interest. In these processes, both soluble and insoluble impurities present in the pregnant solution are incorporated, encapsulated or occluded within the resultant salt crystals.
In addition to mining the salts, solar evaporation of brine from the ocean and from inland lakes as well as the evaporation of brines by open pan or vacuum pan processes are utilized to prepare crystallized salts. The soluble calcium, magnesium and sulfate impurities are present in the source material and normally appear in unacceptably high quantities in the crystallized product thereby requiring further processing to produce a sufficiently pure salt for many industrial purposes. The brine, usually high in organics and carrying bacteria unwanted in food, is often entrapped in solar salt crystals.
One method of treating solar salt to enhance purification is described in U.S. Pat. No. 3,360,343 wherein the crude salt is reduced to a particle size of approximately 400 microns (40 mesh) and contacted with a dilute acid such as hydrochloric or nitric acids. The resultant slurry is agitated, filtered, washed and dried prior to use. Another process utilized in purifying crystaline salts is described in U.S. Pat. No. 4,385,902 wherein a potassium chloride is contacted with an acqueous liquid leaching solution that is saturated with respect to potassium chloride. The solution is percolated through relatively large salt particles, e.g. 28 mesh, with care being taken not to agitate so that the particles are not subdivided into fines. The leaching process has been shown to be effective in reducing soluble impurities in potassium chloride.
A mechanical approach to reducing the impurity level in sodium chloride is described in U.S. Pat. No. 4,094,956 wherein a mechanical process of attrition washing is stated to be effective in removing sulfates from the salt. The method requires a crushing of the salt to expose a major proportion of the sulfates present on the surface of the crystal structure. The reference points out that the coarse crushing of the start material should not break the individual sodium chloride monocrystals. The wash is carried out by strongly agitating the crushed material in a sodium chloride brine having a limited amount of dissolved sulfates therein. The agitation provides interaction between particles to abrade away the surface impurities, causing them to enter into solution, and thereby reduce the level of this undesired constituent.
These known processes for reducing soluble impurity levels in salts, such as sodium and potassium chlorides, are noted for not addressing the problem of insoluble impurities contained within the salt. These processes, designed to operate with relatively large size particles of salt, teach the importance of maintaining particle size during the process so as not to reduce their effectiveness in reducing the soluble impurity level of the treated salt. The effect of these processes on insoluble impurities is limited to surface impurities. As a result, impurities within the salt crystal remain essentially undisturbed. Fine salt particles are either segregated prior to treatment or not beneficiated as a result of these processes.
Accordingly, it is the primary objective of the present invention to provide a method for the purification of salt characterized by the removal of both insoluble and soluble impurities as well as organic materials and entrapped brine. In addition, the present invention is directed to the treatment of small particle size salt to remove both soluble and insoluble impurities therefrom while increasing the crystal size of the purified product so as to exceed that of the start material. Furthermore, the present process is concerned with achieving these goals without the use of acid treating solutions or the substantial energy requirements characteristic of conventional vacuum pan processing thereby providing improved operating costs.